1. Field of the Invention
The present invention relates to a press shoe of a shoe press unit that, together with an opposing surface, forms a nip through which a material web, in particular a tissue web, to be formed or to be treated is guided. It further relates to a tissue machine with at least one such press shoe assigned to a drying cylinder, in particular a Yankee cylinder.
2. Discussion of Background Information
The press shoes hitherto usual have a constant thickness across the width crosswise to the web travel direction.
However, in particular with tissue machines with so-called Yankee drying cylinders, the problem occurs that the nip does not run straight across the width, but has to adapt to the contour of the jacket of the Yankee cylinder in order to achieve, e.g., constant solids content values for the tissue web across the width. The contour of a Yankee cylinder changes as a result of the steam pressure inside and as a result of a temperature expansion of the face cover. If a press shoe is now pressed against the Yankee cylinder to form a nip or press nip, the pressing pressure distribution resulting in the crosswise direction is no longer constant, but deviates upwards or downwards at the edges. The press shoe therefore has to be bent viewed across the width. Although the press shoe is relatively thin and thus features a relatively small moment of bending inertia compared to a suction contact roll, it opposes too great a resistance to a respective bending moment. As a result, the pressing pressure acting on the tissue web edges becomes greater than in the center of the web. In order to combat this problem, attempts have already been made to reduce the thickness of the press shoe evenly across the width, in order to obtain a smaller moment of bending inertia or moment of resistance. However, this measure does not lead to the desired result for production-oriented reasons. The problem could not be completely solved by a different contact pressing in the edge area either, since the contact pressure forces of adjacent contact pressure elements and accordingly the residual errors were too great.